Intrauterine bone marrow transplantation (BMT) holds considerable promise as a therapeutic approach for many congenital immune, hematologic and enzyme deficiencies. Because the early fetal environment is favorable to the development of immune tolerance, mismatched cells should engraft within the fetus without being rejected or causing graft-versus-host disease (GVHD). Our overall goal is to determine the optimal conditions for establishing stable chimerism following in utero BMT. The exact conditions necessary to achieve substantial engraftment have not been systematically studied in terms of donor cell dosage, recipient gestational age, and production and avoidance of GVHD. We propose to conduct allogeneic in utero BMT in an established canine model in order to determine these parameters for safe and successful fetal engraftment. Therefore, the specific aims of this proposal are: Aim 1: Determine the optimal conditions for chimerism following in utero BMT using concentrated hematopoietic stem cells at varying gestational ages and donor cell dosages in fetal canines. The effects of gestational age in pre-immune fetuses on degree and sites of engraftment will be systemically studied. Hematopoietic stem cell (HSC) dose response studies will be done, subsequently varying the number of donor T-cells. We will also evaluate the complications and mortality resulting from over engraftment and/or GVHD. Hypothesis 1A: Fetal canine recipients from 32 to 38 days of gestation will show progressively higher levels of engraftment at progressively earlier gestational ages and higher donor HSC doses using equa to or >10[8] HSC/kg fetal weight. Hypothesis 1B: Donor T-cell dosage must fall within a relatively narrow range in order to achieve sustained engraftment and avoid GVHD, and relatively small incremental adjustments in donor T-cell dosage appreciably alter degree of engraftment. Aim 2: Determine the functional immune capacity of fetal canines. Immunologic assays will be conducted on fetal pups from day 30 through day 42. Hypothesis: Fetal immunologic function increases progressively during mid-gestation in canines and tolerance to foreign antigens decreases after day 40. These studies are designed to increase our understanding of the early events and pathology associated with intrauterine hematopoietic stem cell infusion. This information, in turn, may lead to significant improvements and enhancement of chimerism with a reduction in mortality.